The invention relates to a flotation plant having an air supply means with which air bubbles can be supplied to untreated water supplied to a basin so as to cause floating of flakes suspended in said untreated water by aid of the air bubbles.
Flotation plants in particular operate according to the principle of direct or air flotation, wherein the known direct flotation plants are not very effective since so far only relatively large air bubbles can be produced and delivered to the untreated water. Therefore, the tendency is to use a so-called pressure release flotation which today is used almost everywhere. To dissolve the air amount required, in pressure release flotation, the air is enriched in water and released at a release valve. In a neighboring mixing zone, the partial flow saturated with air is then mixed with untreated water, whereby air bubbles form. As a rule, this mixing zone is not very large, and frequently coalescing of air bubbles occurs there. This in turn results in large air bubbles with the consequence of a reduced efficiency of the flotation plant. Apart from this, this pressure release flotation has the disadvantage that the air bubble introduction can only be effected from the mixing zone, which also reduces the efficiency of the plant.
From DE 26 56 477 A, a flotation arrangement is known in which a gas-permeable fabric mat is used above a bottom depression so as to attain aeration of the liquid contained. However, such fabric mats can yield only relatively large air bubbles, commonly in a size order of 5 mm, so that this embodiment is not suitable for, e.g., separating activated sludge in sewage treatment plants. Moreover, cleaning of the fabric mats is relatively complex.
In EP 250 316 A, a flotation arrangement is described which comprises several compartments, a aeration means, usually ceramic aeration plate means, being provided in a middle diffusion section, whereas in the neighbouring flotation compartment proper no separate aeration means is arranged. In the middle compartment, aeration has to be effected with high speed so as to force the gas, or the air, respectively, downwards and sidewards to the flotation compartments proper. This results in high pressure losses, and the efficiency of this arrangement is relatively low. In this arrangement, primarily a mixing of the gas and water phases is sought, a flotation effect rather being a side-effect thereof. Accordingly, the suspended matter in this arrangement is not moved upwards to the surface of the liquid but is collected on the bottom after having sunk down.
Furthermore, an arrangement for treating slurry water, in particular for separating lacquer sludge, is known from DE 86 29 601 U. For a gas treatment, a porous, non-elastic solid plate is inserted at the entry side of a pressurized air container in this instance. For gas treatment, a high pressure is required; nevertheless, in such porous gas treatment plates, the risk of clogging is high.
Finally, from DE 36 13 665 A a light-liquid separator comprising a reaction compartment is known in which supply ducts for supplying small air bubbles into the liquid are arranged. Here, the air addition mainly is effected for discharging detergents contained in the liquid; this known separator is, however, not suitable to separate supended matter, i.e. substances having a density higher than that of the liquid. In such suspended matter, the natural sedimentation movement must be changed into an upward movement for which purpose so far commonly the so-called pressure release flotation has been used, as has already been mentioned.
It is thus an object of the invention to solve these problems and to provide a flotation plant in which the effect of flotation is improved by simple means.
It is a further object of the invention to provide a flotation plant where an improved introduction of air bubbles into the untreated water is obtained, and also a coalescing of air bubbles is prevented at least mainly.
In the flotation plant according to the invention, the air supply means comprise strip- or plate-like membrane aeration devices for installation in the bottom region and including perforated membranes. In this manner, in the present flotation plant, means known per se are used for introducing air bubbles into the untreated water, i.e. Soto-called strip- or plate-aeration means or diffusers wherein a flexible membrane is mounted over a plate under tension, and is provided with a corresponding number of holes to allow the passage of air under formation of bubbles. Such diffusors are already being used in aeration basins, cf. e.g. WO 95/35156, yet in aeration basins different specific air loads are present as compared to flotation plants. There, e.g., a specific air load is used which is higher by a factor 100 or more as compared to flotation plants, yet for usual aeration purposes also larger air bubbles are delivered: as a rule, the typical diameter of the air bubbles in these conventional diffusers is some mm, e.g. 2 to 3 mm. In the case of flotation plants, however, the air bubbles should, if possible, merely be in the range of a few tenths of a mm, e.g. have a mean size (i.e. a mean diameter) of 0.5 mm.
With strip or plate diffusers, with a corresponding size of air bubbles and a corresponding number of openings or holes in the membranes, substantially better results can be obtained as compared to conventional air and pressure release flotations. By an adapted selection of size and number of the holes, the membranes allow for a wide range of regulation for the introduction of air bubbles, depending on the special use of the flotation plant, specific air loads of, e.g., 0.1 to 5 Nm3/hm2 being possible. With such a specific air load, the flotation plant can be operated at parameters corresponding to those which commonly are found in pressure release flotations; in particular, the rising speed in the flotation compartment may, e.g., be 2 to 10 m3/hm2 and the air/suspended matter ratio may be in the range of from 0.01 to 0.05 Nm3/kgDS (DS-dry substance). The suspended matter content (DS) in the untreated water may, e.g., be from 100 to 20,000 mg/l.
A further advantage of the use of the membrane diffusors mentioned in the flotation plant is that under the given specific air load, also coalescing of air bubbles can be avoided. This is also aided by the fact that the introduction of air can occur distributed over the entire flotation space, which, moreover, enhances the degree of effectiveness when the particles distributed in the liquid float, as well as the enrichment of these particles on the liquid surface, in the flotate layer.
If the operational parameters allow it, it is also very much conceivable to equip the flotation space only partially with the membrane aeration means mentioned. For a particularly efficient flotation it is, however, suitable if substantially the entire bottom of the flotation plant is equipped with aeration means.
To generate fine air bubbles, preferably with a mean diameter of 0.5 mm, generally 0.1 to 0.9 mm, the membrane must have correspondingly small holes, and for this it has proven suitable if the membranes are membranes perforated by micro-needles.
To attain the suitable specific air load in the above-indicated range required for this flotation, it is furthermore advantageous when providing appropriately small holes if the number of holes is from 10 to 100 per cm2 of membrane surface.
The membrane aeration means according to the invention are simple and can be produced, mounted and operated at low cost, and, in principle, they may be constructed in a manner known per se. In this connection, reference is also made to DE 34 41 731 A or to DE 42 40 300 A in addition to the afore-mentioned WO 95/35156 A.
The membranes may simply be made of synthetic material, such as a polycondensate or a polyaddition product. Preferably, the membranes may be made of polyurethane, EPDM, silicone, Viton, polyethylene-trifluoride or polycarbonate. Of course, also other synthetic materials are conceivable, as long as they have the appropriate strength and can be perforated by micro-needles. During such perforation, preferably micro-needles having a diameter of from 0.1 to 1 mm are used, the holes thereby formed in the membranes being practically closed in the unpressurized state of the membranes and opening only at an overpressure, e.g. of from 30 to 80 mbar, on the one side (where the air is supplied).
The invention will now be explained in more detail by way of preferred exemplary embodiments and with reference to the drawings.
FIG. 1 schematically shows a flotation plant in a vertical section;
FIG. 2 shows a somewhat modified flotation plant in a schematic top view;
FIG. 3 shows a schematic cross-section through this flotation plant according to line IIIxe2x80x94III of FIG. 2; and
FIG. 4 shows a schematic cross-section through a plate- or strip-aeration means used in such a flotation plant according to FIGS. 1 to 3.